September 4, 2013 / Unknown Fast mapping

Time of analysis is an important factor for investigations and a determining factor for fast processes .What is the fast and ultra-fast Raman imaging? Everything has to be compared and the terminology doesn't matter.

Minimal time of acquisition of Raman spectrum for each pixel of an image is basically limited by CCD detector operation speed (about: 760 µs – 1 ms) and makes in practice about 5 ms. In Confotec NR500 system we apply two modes for Raman imaging. Alongside with CCD detection mode we apply the mode in which PMT is a receiver and the scanning is realized with galvanic mirrors at a fixed sample. We call this mode Fast mapping, rather modestly, considering that the time of acquisition of each pixel of the image is only 3 µs!And the minimal time of 1001 pixels x 1001 pixels Raman image acquisition is only 3 s!

Some capabilities of Fast mapping mode have been shown at its application to the investigation of Granite Gneiss India sample (the sample has been supplied by Brno University of Technology). Fast mapping mode allows imaging both at relatively strong signals and maximal signals which are several orders weaker, if compared with the Raman signal from silicon. Two dry objectives have been applied for the measurements: 100х and 20х.

As an example the confocal (1AU) megapixel (1002001 pixels) images of Granite Gneiss India sample with total measurement time of 3 seconds are shown in Fig.1. The images in Fig. 1 have been obtained with the objective 100х by scanning of maximum sample size e for the given objective (150 μm х 150 μm) at scanning step of 150 nm.

a – sample surface image in the reflected laser light on the wavelength of 488 nm

b – Raman image of anatase (titanium dioxide) distribution

c – summarized image of anatase distribution relative to the image of the sample surface in the reflected light

d – summarized axonometric image

It is possible to choose only an informative part of the obtained image and realize the scanning of the appropriate part of the object with the same number of pixels. As a result, a more detailed image with higher spatial resolution will be obtained. In Fig. 2a the examples of such megapixel (1002001 pixels) images scanned with the step of 43 nm are given. The informative part of the object can be scanned also in a high sensitivity mode. Thus the image with higher ratio signal/noise will be obtained. Examples of such Fast mapping application for imaging are shown in Fig. 2b.

Figure 2

Figure 2a

Figure 2b

anatasedistribution

laserimage

summed image of anatase distribution relative to the image of sample surface in the reflected light

Anatase spectrum in one of the points of the image (100х objective) is given in Fig.3. The Raman line used for imaging is schematically marked with a rectangle.

Figure 3

Anatase spectrum in one of the points of image 1 (100x objective)

Photoluminescence from the studied material or from sample impurities is a problem interfering with the detection of Raman images. In Confotec NR500 Raman confocal microscope a specially developed system of removing the photoluminescence is applied for the imaging in Fast mapping mode. Its application is successful regardless of photoluminescence appearance- in the form of separate bright spots distorting Raman image, in the form of photoluminescence background in the whole image or in the case of both photoluminescence appearance simultaneously . In Fig. 4a the anatase distribution obtained with the 20x objective in Fast mapping mode with distortions induced by the photoluminescence in the form of separate bright spots is shown. Separate photoluminescence spots are marked with circles for visual demonstration. In Fig. 4b a real anatase distribution is shown after the photoluminescence distortions have been removed. The image of the same sample area but in CCD detection mode (Mapping mode) after use of Baseline Correction function for the photoluminescence removing is shown for comparison in Fig. 4c. Both images have no distortions induced by the photoluminescence. Anatase spectrum in one of the points of sample (20x objective) is shown in Fig. 4d.

d - Anatase spectrum in one of the image pixels (20x objective)

Figure 4Demonstration of removing interfering photoluminescence in the form of separate spots

а- Fast mapping image distorted by the photoluminescence. Some photoluminescence spots are marked with circles for visual demonstration.

c- image of the same sample area obtained in CCD detection mode after use of Baseline correction function for the photoluminescence subtraction

Another example of photoluminescence subtraction that totally distorts Raman image and appear both in the form of separate bright spots and in the form of the photoluminescence background in the whole image is shown in Fig. 5.

High sensitivity. A special mode for increasing the signal/noise ratio at measurement of weak Raman signals.

Combination of high speed, high sensitivity and high resolution allows the fast acquisition of reliable data on distribution of the chemical compound over a sample. When such high-speed and sensitive mode as our Fast mapping is not available the following technique is used for acquisition of chemical compound distribution over a sample: at first a large sample area is measured with a low resolution, and then the informative areas are scanned with higher resolution. However at low resolution a specific compound in a sample may not be found at all, especially if this chemical compound has a form of separate inclusions in a sample.

Raman images acquisition with photoluminescence subtraction .

Channel Reflection Fast Mapping for acquisition of the same high-speed images with high spatial resolution of the same investigated sample areas but in the reflected laser light. Besides the auxiliary information about a sample obtained in the reflected laser light this channel quickly finds the focused areas of the relief surface of a sample.